1. Field of the Invention
This invention relates to a high strength feldspathic porcelain and a method for producing the same. More particularly, the invention relates to high strength feldspathic porcelain having a fine, homogeneous microstructure and being suitable for large high-voltage insulators and acid resisting porcelain, and the invention also relates to a special method for producing such porcelain.
2. Related Art Statement
Two kinds of feldspathic porcelain are used in producing high-voltage insulators; namely, common porcelain and alumina-containing porcelain. The common porcelain consists essentially of quartz-type material, feldspathic material, and clay-mineral material, while the alumina-containing porcelain consists essentially of quartz-type material, feldspathic material, alumina-type material, and clay-mineral material.
FIG. 7 shows the relationship between porcelain strength and the grain size of a starting material mixture therefor the case of common porcelain made of quartz-type material, feldspathic material, and clay-mineral material. The ordinate represents unglazed bending strength of porcelain, while the abscissa represents the percentage by weight of particles with an effective diameter of not greater than 10 .mu.m. As can be seen from FIG. 7, the strength of porcelain tends to increase with the reduction of the particle size of starting material therefor. The reason for such increase of the porcelain strength is in that the use of finer material results in a more homogeneous microstructure in the porcelain. Such tendency is recognized in both the common porcelain and the alumina-containing porcelain.
On the other hand, it is empirically known in the art of porcelain production that the finer the particle size of the starting material, the more likely the occurrence of cracks in the drying and firing processes. In general, when the content of particles with effective diameters of not greater than 10 .mu.m surpasses 85% by weight (to be referred to as % hereinafter), the crack tends to occur in the above processes. Accordingly, the particle size of porcelain material has been controlled so that the content of particles with effective diameters of not greater than 10 .mu.m is less than 85% in both common porcelain and alumina-containing porcelain.
The above "cracks" occurring in the drying or firing process refers to those cracks which can be caused at strained portions of the porcelain by the difference of expansion and shrinkage between the inside and the surface thereof due to temperature differences therebetween during the drying and firing processes. The strained portions include both inside strains caused during kneading and extrusion as a result of difference of particle orientation and density between the inside and the surface of body material and the surface strains caused during cutting the working of the body. Thus, the cracks discussed in the description of the invention refers to cracks formed in the inside and on the surface of the porcelain during the drying and firing processes.
In short, the feldspathic porcelain of the prior art has a shortcoming in that, when the particle size of the starting material is very fine, the risk of crack occurrence becomes high, and such risk is further increased when such porcelain is used in making large high-voltage insulators or the like in which the temperature difference between the inside and the surface of the porcelain is large.
Accordingly, the particle size of the starting material is restricted to be larger than a certain value, which restriction is reflected in a limitation of the homogeneity of the porcelain microstructure and a comparatively low mechanical strength of porcelain. For instance, the three-point bending strength of an unglazed test piece with a 12 mm diameter for insulator porcelain is about 1,000 kg/cm.sup.2 in case of common porcelain and about 1,400 kg/cm.sup.2 in case of alumina-containing porcelain containing 20% of corundum. If the particle size of the starting material is made very fine, cracks occur in the drying process and/or firing process of porcelain production as pointed out above. Thus, for products which require a high mechanical strength, such as large high-voltage insulators, special measures have been taken, for example, adding a large amount of corundum or by applying static hydraulic pressure onto the body for densification followed by carving an insulator out of the densified body. Such special measures result in an increse in the production cost or a complication of the production process.